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Surface subsidence due to reservoir compaction during production has been observed in many large oil fields. Subsidence is most obvious in coastal and offshore fields where inundation by the sea occurs. Well known examples are Wilmington field in California and Ekofisk field in the North Sea. In South Belridge field the Belridge diatomite member of the late Miocene Reef Ridge Shale has proven prone to compaction during production. The reservoir, a high porosity, low permeability, highly compressive rock composed largely of diatomite and mudstone, is about 1000 ft thick and lies at an average depth of 1600 ft. Reservoir compaction within the Belridge diatomite due to withdrawals of oil and water in section 12, T28S, R20E, MDB&M, was noticed after casing failures in producing wells began occurring and tension cracks, enlarged by hydrocompaction after a heavy rainstorm, were observed. Surface subsidence in section 12 has been monitored since April, 1987, through the surveying of benchmark monuments. The average annualized subsidence rate during 1987 was -1.86 ft/yr, -0.92 ft/yr during 1988, and - 0.65 ft/yr during 1989; the estimated peak subsidence rate reached -7.50 ft/yr in July 1985, after l½ yrs of production from the Belridge diatomite reservoir. Since production from the Belridge diatomite reservoir commenced in February, 1984, the surface of the 160 acres producing area has subsided about 12.50 ft. This equates to an estimated reservoir compaction of 30 ft in the Belridge diatomite and an average loss of reservoir porosity of 2.4% from 55.2% to 52.8%. Injection of water for reservoir pressure maintenance in the Belridge diatomite began in June, 1987, and has been effective in mitigating subsidence and repressurizing the reservoir to near-initial pressure. An added benefit of water injection has been improved recovery of oil from the Belridge diatomite by waterflood. The operation of four other water injection projects in South Belridge Field in the Belridge diatomite reservoir suggests that reservoir compaction and surface subsidence is a common occurrence in the field.

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